1. Field of Invention
The present invention relates generally to automatic gain control (AGC) circuits and in particular to an AGC circuit to be used in sonar or radar systems.
2. Prior Art
Various methods have been utilized in the past to reduce the input dynamic range. They include time varying gain (TVG), automatic gain control (AGC), non-linear amplifiers (usually logarithmic) or combinations of these. Usually TVG characteristics are matched to sonar echo returns at various ranges in order to reduce the echo variation at short range where the greatest variation occurs. AGC is utilized to provide additional control at longer ranges where surfaces and bottom returns may cause a rapidly rising reverberation level at the receiver. This rapidly rising reverberation level is due to energy being reflected back to the receiver that was distributed and scattered in space due to the energy impinging on the rough seabed or the uneven water-surface. The primary disadvantage of a TVG is that it has a fixed characteristic (no feedback control) set a priori for some assumed average conditions. It thus falls to the AGC to provide the necessary correction to bring the dynamic range within desirable bounds.
An AGC, unlike a TVG, system, does not have a fixed gain characteristic but utilizes the concept of feedback to limit input variations to desirable bounds. In essence, an AGC performs three functions in order to achieve signal normalization. It estimates the average level of its own output usually by detection and smoothing, it compares this estimated average with a reference set point, and regulates the input in such a manner as to bring the output level and set point into coincidence. These functions may be implemented either digitally or continuously, however in either case the underlying concepts remain the same. The measure of efficiency of a particular AGC system lies in its ability to reduce the dynamic range at its output to a minimum while minimizing the distortion of the desired echo (signal). The AGC characteristic should be insensitive to the level of the input (its echo distortion should not be a function of the AGC gain). In the presence of stationary input noise (usually Rayleigh), the output statistics should be stationary over a wide range of input levels.
Simple AGC systems exhibit a high-pass filter characteristic with a definite low frequency cutoff. The extent to which the AGC is successful in reducing dynamic range while preserving the desired echo depends largely upon the spectral separability of the desired and undesired portions of the input envelope.
FIG. 1 is a diagram of a simple first order linear AGC system similar to that utilized often to correct minor level variations.
The input signal R.sub.i is applied to a variable-gain amplifier 20 which operates as an amplifier whose gain is a function of a control voltage. The output signal from amplifier 20 is applied to an envelope detector 22. The detected envelope is then applied to the negative terminal of summer 24. The summer 24 acts to subtract this envelope from some reference voltage and thus generate an error signal. This error signal is applied to an integrator 26 which acts to smooth out the error signal from the summer. The integrator 26 provides a low-pass filter characteristic with a high-frequency cut-off. Thus the sharp leading-edge of the echo is filtered out of the error signal and no compensation is made for its at the variable-gain amplifier 20. This is the desired result since it is only desired to keep the background level constant, not the echo return.
It is generally preferred that each echo pulse be of a uniform width (exist for a certain period of time) for easy visibility on a scope display. A problem occurs in that after the pulse leading edge (high-frequency spectrum) the automatic gain control (A.G.C.) will compensate the remaining length of the echo pulse (energy concentrated in the low frequency spectrum) at a rate depending on the input background level.
Mathematically the equations determining the system response for a linear AGC are developed as follows. Functionally, the VGA may be considered as an analog multiplier, that is EQU R.sub.o =E R.sub.i
where E is the control voltage to the VGA and where R.sub.i and R.sub.o are the bandpass input and output respectively. Denoting the envelope functions of R.sub.i and R.sub.o by E.sub.i and E.sub.o respectively, the dynamic equations may be readily written as: EQU E.sub.o =E E.sub.i ( 2) EQU E=K.sub.g (R-E.sub.o). (3)
where R is the reference value for the system.
Differentiating (2) and using (3), we obtain ##EQU1## It is apparent from (4) that the device is asymptotically stable at E.sub.o =R and in the absence of an input disturbance will settle to its final value exponentially according to EQU E.sub.o (t)=E.sub.o (o.sup.+)e.sup.-E.sbsp.i.sup.(o.spsp.+.sup.)K.sbsp.g.sup.t +R(1-e.sup.-E.sbsp.i.sup.(o.spsp.+.sup.)K.sbsp.g.sup.t) (5)
Thus the linear AGC system dynamics depend upon the input envelope level E.sub.i. As can be seen in equation 5, the system time constant equals ##EQU2## Thus since the gain K.sub.g is a constant, the larger the input level E.sub.i, the faster the system returns to its reference level. The echo pulse width will then vary according to the input background level.
FIG. 2 illustrates the situations in which an echo pulse occurs above a low and a high background level in a linear AGC. Assume that this AGC system is in a quiesscent state prior to the application of a pulse. The output envelope at a time just prior to the pulse is E.sub.o (o.sup.-)=R, where R is the reference level at the summer 24.
Given a 10R echo pulse as shown in FIG. 2a, (E.sub.o (o.sup.+)=10R), the response of the system will depend on the background level.
FIG. 2b is a graph of the linear AGC response of the circuit in FIG. 1 for a low background level. Since the background level is low, the loop gain is low and thus the time constant is high which means that the AGC will compensate for the pulse at a slow rate.
FIG. 2cis a graph of the linear AGC response for a high background level. Here the echo return is compensated very quickly after the initial rise because the loop gain is high implying a short time constant, the AGC thus reacts very quickly to the pulse, effectively shortening it.
Since radar or sonar echoes may occur against widely varying background levels, the variability of the linear AGC response is a decided disadvantage.